Personally, A Crack In Time was the last game in the series I generally really liked since even though it still has a whole lot of modern Ratchet flaws, it was at least a nice conclusion to an ongoing story and had some interesting gimmicks of its own.
Outside of that, Ratchet has just kind of become a junk food type series. I'm glad it's still running because it offers some nice presentational diversity among Sony's current game slate targeting their primary audience, but ever since the franchise left the PS2 days it's been progressively turning into a whole lot of empty calories; the level design has gotten less ambitious, the action has emphasized glitz over wit, and the writing quality has also deteriorated significantly by sanding down the edges to appeal to more wholesome-minded audiences, even though it consequently ended up making the eponymous pair go from being one of the most interesting buddy comedy duos in games to one of the most mundane. The modern games don't do platforming any more interesting or engaging than its contemporaries, nor does its combat/gunplay really measure up with its peers either. Rift Apart's SSD utilizing gameplay features only really amounted to fairly basic game play features that could be masked as setpiece-worthy elements. The phenomenal production values, comedy gimmick weaponry, and beautiful CG cartoon-like presentation is the one thing holding the series up at this point, and it doesn't exactly serve to elevate the most interesting core. The games are alright, and then they kind of exit the mind after you're done with them.
These aren't horrible numbers, nor are they even necessarily bad numbers when looked at in a vacuum, but I'm not surprised there's a lack of growth here or that it even may be underperforming. The franchise has been pretty unremarkable for some time now and Ratchet himself doesn't really have that household status anymore to carry its shortcomings. It probably doesn't help either that the franchise's value might've been shortchanged (similar to Crash) by the PS4 remake being budget priced. If they want the franchise to be back at the top, the next one feels like it has to do some uprooting of conventions in order to justify how much money is going into them.
